Using data from NASA's Chandra X-ray Observatory, scientists have
reported the possible detection of a binary star system that was later
destroyed in a supernova explosion. The new method they used provides
great future promise for finding the detailed origin of these important
cosmic events.

In an article appearing in the February 14th issue of the journal
Nature, Rasmus Voss of the Max Planck Institute for Extraterrestrial
Physics in Germany and Gijs Nelemans of Radboud University in the
Netherlands searched Chandra images for evidence of a much sought after,
but as yet unobserved binary system - one that was about to go
supernova. Near the position of a recently detected supernova, they
discovered an object in Chandra images taken more than four years before
the explosion.

The supernova, known as SN 2007on, was identified as a Type Ia
supernova. Astronomers generally agree that Type Ia supernovas are
produced by the explosion of a white dwarf star in a binary star system.
However, the exact configuration and trigger for the explosion is
unclear. Is the explosion caused by a collision between two white
dwarfs, or because a white dwarf became unstable by pulling too much
material off a companion star?

Answering such questions is a high priority because Type Ia supernovas
are major sources of iron in the Universe. Also, because of their
nearly uniform intrinsic brightness, Type Ia supernova are used as
important tools by scientists to study the nature of dark energy and
other cosmological issues.

"Right now these supernovas are used as black boxes to measure distances
and derive the rate of expansion of the universe," said Nelemans. "What
we're trying to do is look inside the box."

If the supernova explosion is caused by material being pulled off a
companion star onto the white dwarf, fusion of this material on the
surface of the star should heat the star and produce a strong source of
X-radiation prior to the explosion. Once the supernova explosion
occurs, the white dwarf is expected to be completely destroyed and then
would be undetectable in X-rays. In the merger scenario, the intensity
of X-ray emission prior to the explosion is expected to be much weaker.

Based on the detection of a fairly strong X-ray source at approximately
the position of SN 2007on 4 years before the explosion, Voss and
Nelemans conclude that the data support the scenario where matter is
pulled off a companion star. The small number of X-ray sources in the
field implies that there is only a small chance of an unrelated source
being so close by coincidence. Also, the X-ray source has similar
properties to those expected for fusion on a white dwarf, unlike most
X-ray sources in the sky.

However, in follow-up studies, Voss, Nelemans and colleagues Gijs
Roelofs (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.)
and Cees Bassa (McGill University, Canada) used higher-quality optical
images to better determine the supernova's position. This work, which is
not yet published, shows a small, but significant difference in the
measured positions of the supernova and the X-ray source, suggesting the
source may not be the progenitor.

Follow-up Chandra observations hint that the X-ray object has
disappeared, but further observations are needed to finally decide
whether the source was the progenitor or not.

The team is also applying this new method to other supernovas and has
high hopes that they will eventually succeed in identifying the elusive
cause of at least some of these explosions.

"We're very excited about opening up a new way of studying supernovas,
even though we're not sure that we've seen this particular stellar bomb
before it exploded," said Gijs Roelofs. "We're very confident that
we'll learn a lot more about these important supernovas in the future."

Voss agrees that, even if the X-ray source is not found to be the
progenitor of SN 2007on, the hunt is worth the effort.

"Finding the progenitor to one of these Type Ia supernovas is a great
chase in astronomy right now," he said. "These supernovas are great
tools for studying dark energy, but if we knew more about how they form
they might become even better tools."